package com.castlzl.random.core.snowflake;

import com.castlzl.random.core.IdGenerator;
import lombok.Getter;

/**
 * 雪花算法ID生成器实现
 * 64位ID (42(毫秒)+5(机器ID)+5(业务编码)+12(重复累加))
 * @author leiziliang
 */
public class SnowflakeIdGenerator implements IdGenerator {

    /**
     * 开始时间截 (2023-01-01)
     */
    private final static long START_TIMESTAMP = 1672531200000L;

    /**
     * 机器id所占的位数
     */
    private final static long WORKER_ID_BITS = 5L;

    /**
     * 数据标识id所占的位数
     */
    private final static long DATACENTER_ID_BITS = 5L;

    /**
     * 序列在id中占的位数
     */
    private final static long SEQUENCE_BITS = 12L;

    /**
     * 支持的最大机器id
     */
    private final static long MAX_WORKER_ID = ~(-1L << WORKER_ID_BITS);

    /**
     * 支持的最大数据标识id
     */
    private final static long MAX_DATACENTER_ID = ~(-1L << DATACENTER_ID_BITS);

    /**
     * 机器ID向左移12位
     */
    private final static long WORKER_ID_SHIFT = SEQUENCE_BITS;

    /**
     * 数据标识id向左移17位(12+5)
     */
    private final static long DATACENTER_ID_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS;

    /**
     * 时间截向左移22位(5+5+12)
     */
    private final static long TIMESTAMP_LEFT_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS + DATACENTER_ID_BITS;

    /**
     * 生成序列的掩码
     */
    private final static long SEQUENCE_MASK = ~(-1L << SEQUENCE_BITS);

    /**
     * 工作机器ID(0~31)
     */
    @Getter
    private final long workerId;

    /**
     * 数据中心ID(0~31)
     */
    @Getter
    private final long datacenterId;

    /**
     * 毫秒内序列(0~4095)
     */
    private long sequence = 0L;

    /**
     * 上次生成ID的时间截
     */
    private long lastTimestamp = -1L;

    public SnowflakeIdGenerator(long workerId, long datacenterId) {
        if (workerId > MAX_WORKER_ID || workerId < 0) {
            throw new IllegalArgumentException(String.format("Worker Id can't be greater than %d or less than 0", MAX_WORKER_ID));
        }
        if (datacenterId > MAX_DATACENTER_ID || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("Datacenter Id can't be greater than %d or less than 0", MAX_DATACENTER_ID));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    @Override
    public synchronized long nextId() {
        long timestamp = timeGen();

        // 如果当前时间小于上一次ID生成的时间戳，说明系统时钟回退过这个时候应当抛出异常
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(
                    String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

        // 如果是同一时间生成的，则进行毫秒内序列
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & SEQUENCE_MASK;
            // 毫秒内序列溢出
            if (sequence == 0) {
                // 阻塞到下一个毫秒,获得新的时间戳
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            // 时间戳改变，毫秒内序列重置
            sequence = 0L;
        }

        // 上次生成ID的时间截
        lastTimestamp = timestamp;

        // 移位并通过或运算拼到一起组成64位的ID
        return ((timestamp - START_TIMESTAMP) << TIMESTAMP_LEFT_SHIFT)
                | (datacenterId << DATACENTER_ID_SHIFT)
                | (workerId << WORKER_ID_SHIFT)
                | sequence;
    }

    /**
     * 阻塞到下一个毫秒，直到获得新的时间戳
     *
     * @param lastTimestamp 上次生成ID的时间截
     * @return 当前时间戳
     */
    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    /**
     * 返回以毫秒为单位的当前时间
     *
     * @return 当前时间(毫秒)
     */
    protected long timeGen() {
        return System.currentTimeMillis();
    }

    @Override
    public String getName() {
        return "snowflake";
    }
}